This invention relates generally to integrated semiconductor lasers. Specifically, the present invention relates to optically pumped vertical cavity surface emitting lasers (VCSELs).
A VCSEL is a semiconductor laser consisting of a semiconductor layer of optically active material, such as gallium arsenide or indium gallium arsenide or the like, sandwiched between mirrors formed of highly-reflective layers of metallic material, dielectric material, epitaxially-grown semiconductor dielectric material or combinations thereof, most frequently in stacks. As is conventional, one of the mirror stacks is partially reflective so as to pass a portion of the coherent light built up in the resonating cavity formed by the mirror stack/active layer sandwich.
Laser structures require optical confinement in a cavity and carrier confinement to achieve efficient conversion of pumping electrons into stimulated photons through population inversion. The standing wave of reflected electromagnetic energy in the cavity has a characteristic cross-section giving rise to an electromagnetic mode. A desirable electromagnetic mode is the single fundamental mode, for example, the HE.sub.11 mode of a cylindrical waveguide. A single mode signal from a VCSEL is easy to couple into an optical fiber, has low divergence and is inherently single frequency in operation.
The total gain of a VCSEL must equal its total loss in order to reach the lasing threshold. Unfortunately, due to the compact nature of VCSELs, the gain media is quite limited. This limitation results in a requirement that for efficient VCSELs, the mirrors have a reflectivity of greater than approximately 99.5 percent. This requirement is much more difficult to meet in long wavelength VCSELs than in short wavelength VCSELs since the mirrors cannot be grown in the same epitaxial step as the active region. For example, in a 980 nanometer GaAs VCSEL the mirrors can be grown using alternating layers of GaAs and AlGaAs. Since the refractive index difference between these two materials is 0.6, very few layers are required to form a suitable mirror. An analogous mirror design for a 1300 or 1550 nanometer VCSEL would use alternating layers of InP and InGaAsP. In this case, however, the refractive index difference is approximately 0.23. As a result, an InP/InGaAsP mirror must be much thicker to achieve the same reflectivity as a GaAs/AlGaAs mirror. Increasing thickness, however, does not work in practice since both absorption and diffraction losses increase as well, ultimately limiting the maximum achievable reflectivity.
Therefore, in order to form a useful long wavelength VCSEL, the mirrors must be formed of either evaporated dielectrics or lattice mismatched semiconductors. FIGS. 1 and 2 illustrate two possible mirror combinations described in the prior art. Both structures use at least one wafer-fused GaAs/AlAs mirror 2 which has a larger index difference than InP/InGaAsP. Wafer fusion is a known technique whereby semiconductors of differing lattice constants can be atomically joined, simply by applying mechanical pressure and heat. The structure shown in FIG. 1 uses an electrically insulating dielectric mirror 3 as the top mirror while the structure shown in FIG. 2 uses a second wafer-fused GaAs/AlGaAs mirror 2 as the top mirror.
The VCSEL structures shown in FIGS. 1 and 2 suffer from several problems associated with electrical injection of charge carriers into the active region. The structure of FIG. 1 has an insulating dielectric top mirror 3, thus requiring a metal ring contact 4 and injection around dielectric mirror 3 along the injection path 5. This contacting and injection scheme results in a complicated fabrication procedure. The structure of FIG. 2 uses injection through a conducting top mirror 2 with a metal contact 6. Mirror 2, however, is typically resistive and introduces significant resistive heating. Since the optical efficiency of materials such as InP and InGaAsP are known to degrade rapidly with temperature, the resistive heating will limit the device's output power. Finally, the structures of both FIGS. 1 and 2 as well as any other electrically injected VCSELs require p and n dopants inside of the optical cavity. The dopants introduce further optical loss which ultimately limits the output power.
An alternative to electrical pumping is optical pumping. Optical pumping avoids complex fabrication, resistive heating, and dopant-induced losses. One approach which has been used on a short wavelength VCSEL operating at 860 nanometers was described by McDaniel et al. in an article entitled Vertical Cavity Surface-Emitting Semiconductor Laser with CW Injection Laser Pumping, IEEE Photonics Tech. Lett., 2 (3) (Mar. 1990) 156-158. The authors used an array of in-plane semiconductor lasers as a pump source for a single short wavelength VCSEL. In a different approach to optical pumping, Lin et al. demonstrated a long wavelength VCSEL structure consisting of 30 pairs of compressive strained wells and tensile strained barriers and Si/SiO.sub.2 dielectric mirrors optically pumped with a mode-locked Ti-sapphire laser. Photopumped Long Wavelength Vertical-Cavity Surface-Emitting Lasers Using Strain-Compensated Multiple Quantum Wells, Appl. Phys. Lett. 64 (25) (20 Jun. 1994) 3395-3397. Neither of the above approaches, nor any other approach using an in-plane semiconductor laser, dye laser, or solid-state laser pump, is practical for commercial VCSELs. Practical commercial VCSELs must be manufacturable and testable on a wafer scale in order to have a clear commercial advantage over in-plane semiconductor lasers.
From the foregoing, it is apparent that what is needed is a compact optically pumped long wavelength VCSEL which is manufacturable and testable on a wafer scale.